Refactor and modernize namespace declarations

Move all unit tests to namespace nfd::tests

Delete unused header core/algorithm.hpp

Change-Id: I5591f0c5f3bb5db67f8b45fae95471f8a555ca68
diff --git a/tests/daemon/mgmt/strategy-choice-manager.t.cpp b/tests/daemon/mgmt/strategy-choice-manager.t.cpp
index 3e42d1c..71763bb 100644
--- a/tests/daemon/mgmt/strategy-choice-manager.t.cpp
+++ b/tests/daemon/mgmt/strategy-choice-manager.t.cpp
@@ -31,8 +31,7 @@
 
 #include <ndn-cxx/mgmt/nfd/strategy-choice.hpp>
 
-namespace nfd {
-namespace tests {
+namespace nfd::tests {
 
 class StrategyChoiceManagerFixture : public ManagerFixtureWithAuthenticator
 {
@@ -239,5 +238,4 @@
 BOOST_AUTO_TEST_SUITE_END() // TestStrategyChoiceManager
 BOOST_AUTO_TEST_SUITE_END() // Mgmt
 
-} // namespace tests
-} // namespace nfd
+} // namespace nfd::tests